An exhaust manifold, front pipe, and center pipe, or other exhaust system member of an automobile carries high temperature exhaust gas exhausted from an engine, so the material forming the exhaust member is required to have oxidation resistance, high temperature strength, heat fatigue characteristics, and various other properties.
In the past, the general practice was to use cast iron for the automobile exhaust members, but from the viewpoint of toughening of exhaust-gas regulations, improvement of engine performance, lightening of the weight of the chassis, etc., stainless steel exhaust manifolds have come into use. The exhaust gas temperature differs depending on the car model, but in recent years, the temperature has mostly been 750 to 900° C. A material having excellent high temperature strength and oxidation resistance in an environment where the material is used for a long time in this temperature region has been demanded.
Among stainless steels, austenitic stainless steel is superior in heat resistance and workability, but has a large coefficient of heat expansion, so is susceptible to heat fatigue breakage when used for a member such as an exhaust manifold which is repeatedly heated and cooled.
On the other hand, ferritic stainless steel has a smaller coefficient of heat expansion compared with austenitic stainless steel, so is superior in heat fatigue characteristics and scale peeling resistance. Further, compared with austenitic stainless steel, it does not contain Ni, so the cost of the material is low and it is in general use. However, ferritic stainless steel is lower than austenitic stainless steel in high temperature strength, so technology has been developed for improving the high temperature strength. For example, there are SUS430J1 (Nb steel), Nb—Si steel, and SUS444 (Nb—Mo steel). These improve the high temperature strength by basically addition of Nb and by addition of Si and Mo. Among these, SUS444 has about 2% of Mo added to it, so is highest in strength, but has the problems that it is inferior in workability and contains a large amount of expensive Mo, so is high in cost.
Various additive elements are being studied in addition to the above alloys. Japanese Patent Publication (A) No. 2006-37176, International Publication WO2003/004714, Japanese Patent No. 3468156, and Japanese Patent No. 3397167 disclose the technology of addition of Cu or Cu—V. Regarding the addition of Cu in Japanese Patent Publication (A) No. 2006-37176, addition of 0.5% or less is being studied for improvement of the low temperature toughness. It is not addition from the viewpoint of the heat resistance. International Publication WO2003/004714, Japanese Patent No. 3468156, and Japanese Patent No. 3397167 disclose the technology of utilizing precipitation hardening by Cu precipitates to improve the high temperature strength in the 600° C. or 700 to 800° C. temperature region. Japanese Patent Publication (A) No. 2006-37176, International Publication WO2003/004714, Japanese Patent Publication (A) No. 9-279312, Japanese Patent Publication (A) No. 2000-169943, and Japanese Patent Publication (A) No. 10-204590 disclose steel containing B as ferritic stainless steel superior in high temperature characteristics. The prior art for improvement of the high temperature strength using the addition of Cu utilizes Cu precipitates, but when Cu precipitates are exposed to a high temperature over a long time, coarsening rapidly occurs due to agglomeration and merger of precipitates, so there is the problem that the precipitation strengthening ability ends up remarkably falling. When undergoing the heat cycle accompanying the starting and stopping of the engine like an exhaust manifold, the danger arises that the high temperature strength will remarkably fall at the stage of long term use and heat fatigue breakage will occur.
Further, depending on the engine structure, sometimes the temperature of the exhaust gas will rise up to about 900° C. As described in International Publication WO2003/004714, with the addition of Cu or the composite addition of Cu—V, the yield strength at 900° C. does not reach the SUS444 level, so sufficient reliability is not obtained as an exhaust part. In past discoveries, B was added to improve the workability and improved the grain boundary strength due to grain boundary segregation so as to improve the secondary workability. The effect on the high temperature characteristics was not clear.